For more than half a century, the cathode ray tube (CRT) has been the principal electronic device for displaying visual information. The widespread usage of the CRT may be ascribed to the remarkable quality of its display characteristics in the realms of color, brightness, contrast and resolution. One major feature of the CRT permitting these qualities to be realized is the use of a luminescent phosphor coating on a transparent faceplate.
Conventional CRT's, however, have the disadvantage that they require significant physical depth, i.e., space behind the actual display surface, making them bulky and cumbersome. They are fragile and, due in part to their large vacuum volume, can be dangerous if broken. Furthermore, these devices consume significant amounts of power.
The advent of portable computers has created intense demand for displays which are light-weight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional CRT, there has been significant interest in efforts to provide satisfactory so-called "flat panel displays" or "quasi flat panel displays," having comparable or even superior display characteristics, e.g. , brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing fiat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles is possible, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, thin film field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of pointed, thin-film, cold field emission cathodes in combination with an anode comprising a phosphor-luminescent screen. The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appear to be promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,857,161, "Process for the Production of a Display Means by Cathodoluminescence Excited by Field Emission," issued 15 Aug. 1989, to Michel Borel et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
One of the problems yet to be overcome is the ability to fabricate emitter microtips which provide constant display intensity despite processing variations. In the current technology, field emission cathodes are formed of a multiplicity of arrays, wherein each array includes a small cluster of microtips, illustratively twenty-five microtips. At each array a plurality of fixed-diameter holes are etched; the diameter of the hole influences the emitter microtip form and functionality. This emitter hole size is frequently referred to as the critical dimension. As the critical dimension of a hole changes due to processing variations, the emission from the microtip associated with that hole varies as well. This causes changes in the electron flux to the anode, resulting in variations in the display intensity. The range of emitter hole critical dimensions for which acceptable microtip emission is achieved is quite narrow, and the transition between emission and no-emission is sharply defined.
Current solutions to this variable display intensity problem involve process control and equipment design techniques that attempt to prevent the etched critical dimension of the emitter array matrix from drifting from an optimal size. Acceptable display performance is limited by the emitter hole processing equipment capability, as any variation in the critical dimension across the display will result in a change in the emission characteristics of the display.
In view of the above, it is clear that there exists a need to develop an emitter structure which provides an improvement in the emitter flux uniformity over processing variations than is currently known in the art.